Abstract:
Pollution of surface water with harmful chemicals and eutrophication with excess nutrients are
serious environmental concerns. Eutrophication is a problem that alters the ecological integrity of
any water resources at global, regional and local scale including Ethiopia. It is resulted primarily
due to phosphorus and nitrogen that exported and loaded from agricultural fields. This lends the need
of knowing the nutrient loading and transport mechanism that will occur with spatial and temporal e
xtent. Thus, effective information regarding the nutrient load and transport mechanisms ar e importa
nt to hydrologists, water use planners, watershed management and decision makers for sustainable
water resource projects and planet ecosystem.
Consequently, this study was aimed at Modeling Phosphorus and Nitrogen loading and its transport
pathways and to identify the prone sub basin that were responsible for a significant Phosphorus and
Nitrogen load in Didessa catchment. Soil and Water Assessment Tool (SWAT) model was used to dete
rmine the nutrient loading and its transport pathways. The input data used were digital elevation
model, land use/land cover map, soil map, stream flow data and metrological data. The data were obt
ained from Ministry of water, irrigation and electricity. Simulation of SWAT was used in identifying t
he most vulnerable sub basin to the hydrological process. The model was calibrated and validated usi
ng the Stream flow of Didessa near Arjo gaging station. Sensitivity analysis shows curve number,
ALPHA-BNK and CH-K2 are the most sensitive top three parameters.
The model was calibrated using stream flow data from 2000 to2008 and validated from 2009 to 2014.
The R2
and NSE values were used to examine model performance and the result indicates 0.84 and
0.80 to R2
and 0.65 and 0.54 to NSE during calibration and validation respectively. Following this,
the pathways of Phosphorus and Nitrogen were identified and found that the organic form of
Phosphorus and Nitrogen was the dominant exporting mechanism in the study area and accounts
around 58.89% and 82.26% of the total path. For the all forms of Phosphorus and Organic N,
surface run off was the dominant means of transport agent. The three ways by which NO3 transport
was found as surface run off, lateral flow and through percolation to ground water. The average
annual surface run off contribution in study duration was found as 774.13 (mm). The average annual
loading of total Phosphorus and total Nitrogen were identified as 20.01kg/h and 22.22kg/h in the
study area during study period. The most three annual surface runoff contributing, sub basins are 11,
23 and 5 whereas sub basins 11, 3 and 5 contributes the highest sediment yield. The sub basin
17,23,3 and 16, 17, 22 were identified as the three highest loading of total P and total N respectively.
